Comparison of AlEt_2Cl and ZnCl_2 supported on silica gel as catalysts of Diels-Alder reactions. Influence of the nature of the dienophile

摘要

Non-modified silica gel, silica gel treated with AlEt_2Cl, and ZnCl_2 supported on silica gel are tested as catalysts in the Diels-Alder reactions of cyclopentadiene (1) with methyl acrylate (2a), (―)-menthyl acrylate (2b) and acrylonitrile (2c). In general, supported catalysts are more efficient than non-modified silica gel. Two different kinds of silica gel are tested and the amount of aluminium incorporated when they are treated with AlEt_2Cl changes from one to another, which influences the chemical yield but not the selectivity of the reactions. In general, aluminium catalysts are more efficient than ZnCl_2-supported ones in the reactions of acrylates, but the latter solid is a better catalyst in the reactions of acrylonitrile (2c). Differences in acid strength and hardness of the catalytic sites are invoked to account for these results.